N A Rutter

Importance of low-angle grain boundaries in YBa2Cu3O7-delta coated conductors

Over the past ten years the perception of grain boundaries in YBa2Cu3O7−δ conductors has changed greatly. They are now not a problem to be eliminated, but an inevitable and potentially favourable part of the material. This change has arisen as a consequence of new manufacturing techniques which result in excellent grain alignment, reducing the spread of grain boundary misorientation angles. At the same time there is considerable recent evidence which indicates that the variation of properties of grain boundaries with mismatch angle is more complex than a simple exponential decrease in critical current. This is due to the fact that low-angle grain boundaries represent a qualitatively different system to high-angle boundaries. The time is therefore right for a targeted review of research into low-angle YBa2Cu3O7−δ grain boundaries. This article does not purport to be a comprehensive review of the physics of grain boundaries as found in YBa2Cu3O7−δ in general; for a broader overview we would recommend that the reader consult the comprehensive review of Hilgenkamp and Mannhart (2002 Rev. Mod. Phys. 74 485). The purpose of this article is to review the origin and properties of the low-angle grain boundaries found in YBa2Cu3O7−δ coated conductors both individually and as a collective system.

Percolation modelling for highly aligned polycrystalline superconducting tapes

Surface and bulk texture measurements have been carried out on highly aligned NiFe tapes, suitable for use as coated conductor substrates. Data from small-area electron backscatter diffraction measurements are compared with those from bulk x-ray analysis in the development of a two-dimensional percolation model, and the two are shown to give very similar results. No evidence of grain-to-grain correlation is found. The model is then developed to assess how the properties of a superconducting layer grown epitaxially on buffered tapes will depend on parameters such as sample size, grain size and the extent of grain alignment.

Benefits of current percolation in superconducting coated conductors

The critical currents of coated conductors fabricated by metal-organic deposition (MOD) on rolling-assisted biaxially textured substrates (RABiTS) and by pulsed laser deposition (PLD) on ion-beam assisted deposition (IBAD) templates have been measured as a function of magnetic field orientation and compared to films grown on single crystal substrates. By varying the orientation of magnetic field applied in the plane of the film, we are able to determine the extent to which current flow in each type of conductor is percolative. Standard MOD/RABiTS conductors have also been compared to samples whose grain boundaries have been doped by diffusing Ca from an overlayer. We find that undoped MOD/RABiTS tapes have a less anisotropic in-plane field dependence than PLD/IBAD tapes and that the uniformity of critical current as a function of in-plane field angle is greater for MOD/RABiTS samples doped with Ca.

Texture development in long lengths of NiFe tapes for superconducting coated conductor

A highly oriented cubic texture (full width at half maximum <10°) has been formed in long length NiFe tapes. The X-ray diffraction (XRD), electron back-scattered patterns (EBSP) and optical microscopy (OM) techniques have been used in assessing the surface and volume texture and also the surface morphology of these kilometer long NiFe substrates. This texture was formed under a range of conditions including dynamic annealing in a reduced atmosphere and static annealing in hydrogen and in vacuum. Heat treatment for excessive times in vacuum tends to roughen the surface and should be avoided. Mechanical polishing can induce an additional undesirable texture, but electropolishing gives smooth tapes with good texture.

A new method for decreasing transport ac losses in multifilamentary coated superconductors

Numerical modelling was used to investigate the effect on transport ac losses of decoupling the filaments in superconducting YBCO coated conductors by magnetic screening. The different configurations of multifilament geometry with filaments covered with layers of a soft iron with non-linear B(H) characteristic considered were modelled. Calculations of contribution of the individual filaments to ac loss were conducted for selected configurations. A comparison of the calculated ac loss results in the multifilamentary conductors with a perfect inter-filamentary decoupling was provided. Since the multifilamentary coated conductors appear to be a favourable solution for low ac losses applications, the factors which affect the percolative nature of current flow in superconducting coated conductors, such as grain size and sample dimensions, have also been presented.

Superconducting-magnetic heterostructures as a new method of decreasing transport AC losses in multifilamentary and coated superconductors

Abstract Numerical modelling was used to investigate the effect of decoupling the filaments in superconducting tapes by magnetic screening on transport AC losses. Both multifilamentary Bi-2223 tapes and multifilamentary YBCO coated conductors were modelled. Different configurations of multifilament geometry with filaments covered with layers of a soft iron with non-linear B ( H ) characteristic were considered. Calculations of contribution of the individual filaments to AC loss were conducted for selected configurations. A comparison of the calculated AC loss results in the multifilamentary conductors with a perfect inter-filamentary decoupling was provided. The obtained AC loss level of modelled 55 “coupled” filaments agrees at the critical current value with the experimental results achieved for 55-filamentary Bi-2223 tape. Since the multifilamentary YBCO coated conductors appear to be a favourable solution for low AC losses applications, the factors which affect the percolative nature of current flow in superconducting coated conductors, such as grain size and sample dimensions, have also been studied in order to predict critical current density variations.

The processing and properties of single grain Y?Ba?Cu?O fabricated from graded precursor powders

The preparation of single grain, Y?Ba?Cu?O (YBCO) bulk superconductors by top-seeded melt-growth (TSMG) usually involves precursor powders that contain a uniform distribution of the constituent YBa2Cu3O7?? (Y-123) and Y2BaCuO5 (Y-211) phase compounds. However, it has been observed that the concentration of Y-211 particles in the fully melt processed superconducting bulk increases significantly with distance from the seed, which results in a degradation of superconducting properties towards the edge and bottom of the sample. Here we investigate the effect of preparing bulk YBCO superconductors by TSMG using spatially graded Y-211/Y-123 precursor powder. The graded precursor bulks were prepared with a maximum composition of 40?wt% Y-211 in the vicinity of the seed, which decreased to 30?wt% and then 20?wt% towards the bottom and edge of the green body. Standard samples were melt processed from precursor powders containing 30?wt% Y-211 to enable comparison. The field trapping ability, Tc and Jc, of three graded and two standard samples were investigated and compared statistically. The distribution of Y-211 particles along different growth directions of the samples was analysed, and any crystallographic misorientation was investigated. The observed distribution of Y-211 particles in YBCO is explained qualitatively by trapping/pushing theory, and its correlation with the superconducting properties of the melt processed bulk samples has been analysed. Finally, the practical feasibility of the graded technique is evaluated.

Computer simulation of current percolation in polycrystalline high-temperature superconductors

YBCO-coated conductors were modelled in a computer simulation using a resistor network concept, with the resistors representing the grain boundaries. Dissipation above the critical current, accompanied by flux penetration into the grain boundaries, was described by a linear (flux-flow) resistivity. The model allowed calculation of the combined percolation of current and magnetic flux. Current-voltage data showed scaling in agreement with percolation theory for two-dimensional systems. The influence of grain alignment and electromagnetic parameters on conductor performance was investigated.

Influence of magnetic materials on the transport properties of superconducting composite conductors

Magnetic materials can help to improve the performance of practical superconductors on the macro/microscale as magnetic diverters and also on the nanoscale as effective pinning centres. It has been established by numerical modelling that magnetic shielding of the filaments reduces ac losses in self-field conditions due to decoupling of the filaments and, at the same time, it increases the critical current of the composite. This effect is especially beneficial for coated conductors, in which the anisotropic properties of the superconductor are amplified by the conductor architecture. However, ferromagnetic coatings are often chemically incompatible with YBa2Cu3O7 and (Pb,Bi)2Sr2Ca2Cu3O9 conductors, and buffer layers have to be used. In contrast, in MgB2 conductors an iron matrix may remain in direct contact with the superconducting core. The application of superconducting–magnetic heterostructures requires consideration of the thermal and electromagnetic stability of the superconducting materials used. On the one hand, magnetic components reduce the critical current gradient across the individual filaments but, on the other hand, they often reduce the thermal conductivity between the superconducting core and the cryogen, which may cause the destruction of the conductor in the event of thermal instability. A possible nanoscale method of improving the critical current density of superconducting conductors is the introduction of sub-micron magnetic pinning centres. However, the volumetric density and chemical compatibility of magnetic inclusions has to be controlled to avoid suppression of the superconducting properties.

Modelling of orientation relations in 2-D percolative systems of buffered metallic substrates for coated conductors

The crystallographic alignment of various layers of high T/sub c/ coated conductors has been studied in detail using X-Ray Diffraction (XRD) and Electron BackScatter Diffraction (EBSD). Several possible configurations of buffer layers on NiFe substrates are considered, including metallic buffers (Pd/Ag) and native oxides (NiFe/sub 2/O/sub 4/) as well as the more widely studied ceramic buffer route(CeO/sub 2//YSZ). The surface texture data (EBSD) was compared with bulk texture measurements (XRD) in order to assess the relationship between them. The percolative properties of the grain structure were investigated using a two dimensional hexagonal grain model, which considers both in-plane and out-of-plane grain boundary misorientations. The model was extended to consider critical currents of macroscopic samples. Factors which affect the percolative nature of current flow in superconducting coated conductors, such as grain size and tape dimensions, have also been studied.